1. FIELD OF THE INVENTION
The present invention relates to a process of producing an alkali permanganate. More particularly, the invention relates to a process of producing a high-pure alkali permanganate at a high yield on an industrial scale without need of complicated operation by preparing a slurry of a tetra-valent manganese oxide and/or an alkali penta-valent manganate having a total caustic alkali concentration of 10 to 25% by weight and subjecting the slurry to an electrolytic oxidation.
2. DESCRIPTION OF THE PRIOR ART
As a conventional typical process of producing an alkali permanganate on an industrial scale, there is a process in which a manganese (IV) oxide is subjected to an oxidation roasting together with a caustic alkali to form an alkali manganate (VI) and after leaching with water, the alkali manganate is electrolytically oxidized.
However, the process encounters various difficulties in many points such as the cost of equipment, the power required, and the labor since, in the process, indirect heating is required for controlling the rate of CO.sub.2 gas absorption by the caustic alkali which makes the apparatus complicated, and also the raw materials stick to the inside of the roasting furnace, which reduces the conversion but also the operation efficiency. Furthermore, the aforesaid process is operable only when the caustic alkali employed is caustic potash and since the conversion is reduced greatly when caustic soda is used as the caustic alkali, the process has not been industrially practiced in the latter case.
On the other hand, there is also proposed a process in which the alkali manganate (VI) prepared by fusing manganese (IV) oxide and a caustic alkali together with an oxidizing agent such as an alkali nitrate is converted into an alkali permanganate by an electrolytic oxidation, by an oxidation with an oxidizing agent, or by disproportionation.
However, the aforesaid process wherein the alkali manganate (VI) prepared by fusing manganese oxide and a caustic alkali together with an oxidizing agent encounters the following problem. That is, the first fusion reaction is fundamentally shown by the reaction formula EQU MnO.sub.2 + MNO.sub.3 + 2MOH .fwdarw. M.sub.2 MnO.sub.4 + MNO.sub.2 + H.sub.2 O (1)
wherein M represents Na or K.
Even if, however, the fusion reaction is carried out for several hours at temperature higher than 500.degree. C, the conversion to the alkali manganate (VI) is low. In the proposed process the alkali manganate (VI) thus-formed is separated usually from the slurry product and is subjected to an electrolytic oxidation to form an alkali permanganate, but since, as descirbed above, the conversion in the first reaction is low, the yield for the alkali permanganate from the manganese oxide is low and thus the process is not practiced for industrial purposes.
Moreover, a process is known in which the slurry of the alkali manganate thus-prepared is oxidized as it is by an oxidizing agent such as chlorine, but in the process it is difficult to regenerate and reuse the excessive alkali used in the process and further the conversion to the alkali permanganate is unsatisfactory.
Furthermore, there is also provided a process in which manganese (IV) oxide in a slurry state is electrolytically oxidized in a heated aqueous caustic alkali solution having a high concentration of about 30 to 40 % by weight to form an alkali manganate (VI) as shown in the following reaction formula ##STR1## wherein M represents Na or K, the alkali manganate is dissolved in a diluted aqueous caustic alkali solution having a concentration of about 10% by weight in a separate step and subjected to an electrolytic oxidation to form an alkali permanganate as shown by the following reaction formula ##STR2## wherein M has the same meaning as above.
It is well known that as the process employs an aqueous caustic alkali solution having a high concentration the rate of reaction is high. However, on the other hand, the process has such a fault that as clear from reaction formula (2), the alkali manganate only is formed in the first step and thus the second step shown in reaction formula (3) is necessary for obtaining the alkali permanganate and, in addition, a concentration step of the aqueous caustic alkali solution is required for circulating the MOH by-produced in the step of reaction formula (3) to the step of reaction formula (2). That is, since the process requires complicated steps, such as the two electrolytic oxidation steps, the concentration step, the circulation step, etc., which results in increasing the production cost for the product, the process is disadvantageous for industrial practice.
After all, these processes as described above are confined to a proposition only at present and have various problems in the industrial practice of them since there are many unknown points in the reaction mechanisms of the processes and also the yield for the final aimed product, alkali permanganate is low, which is the fatal fault of these processes.